1. Field of the Invention
The present invention relates to supported catalysts containing a platinum metal and their use in processes for preparing diaryl carbonates by reaction of aromatic hydroxy compounds with carbon monoxide and oxygen, which are characterized in that pulverulent or shaped supports which can act as redox catalysts under reaction conditions are used.
2. Description of the Related Art
It is known that organic carbonates can be prepared by oxidative reaction of aromatic hydroxy compounds with carbon monoxide in the presence of a noble metal catalyst (German Offenlegungsschrift 28 15 512). The noble metal preferably used is palladium. In addition, a cocatalyst (e.g. manganese or cobalt salts), a base, a quaternary salt, various quinones or hydroquinones and desiccants can be used. The reaction can be carried out in a solvent, preferably methylene chloride.
For economically carrying out this process, not only the activity and the selectivity but also the effective recovery of the noble metal catalyst are of decisive importance: for one thing, the noble metal catalyst represents a considerable cost factor. Losses of noble metal catalyst have to be replaced at high cost. Furthermore, no residues of noble metal catalyst may remain in the product. For the process of oxidative carbonylation of aromatic hydroxy compounds to give diaryl carbonates, the economical and efficient recovery of homogeneous catalysts has hitherto not been described. A noble metal catalyst can be separated from a liquid reaction mixture with less effort, e.g. by filtration or centrifugation, if heterogeneous catalysts, e.g. supported catalysts, are used.
For preparing supported catalysts, generally suitable materials are known. Depending on the type of process, use is made of supports having a high internal surface area, for example alumina, magnesia, activated carbon or silica having more than 50 m.sup.2 of surface area per gram, or supports having surface areas around 5 m.sup.2 /g and correspondingly larger pore radii, for example carbon black, titanium dioxide, iron oxide or zinc oxide, or coarse-grained supports, for example silicon carbide and corundum (Ullmanns Enzyklopadie der technischen Chemie, 3rd edition, Berlin/Munich 1957, Volume 9, p. 263 ff.). Essentially, it is possible to use both synthetic materials such as activated aluminas, silica gels, silicates, titanium dioxides or activated carbons and also materials from natural sources, for example pumice, kaolin, bleaching earths, bauxites, bentonites, kieselguhr, asbestos or zeolites.
In EP 572 980, EP 503 581 and EP 614 876, use is made of supported noble metal catalysts containing 5% of palladium on carbon supports. However, according to our own studies, such supported catalysts give only very unsatisfactory conversions, if any, so that these too are not suitable for an economical process.
In JP 01/165 551 (cited according to C.A. 112 (1990), 76618j) it is mentioned that, for the preparation of aromatic carbonates, palladium or palladium compounds such as palladium acetylacetonate can be used in combination with alkali metal (alkaline earth metal) iodides or onium iodides such as tetrabutylammonium iodide and at least one zeolite.
JP 04/257 546 and JP 04/261 142 describe, in one example each, a supported catalyst for preparing aromatic carbonates in which granulated silicon carbide is used as support material for a supported catalyst in a distillation column. Although the relevant examples are carried out under drastic conditions (high pressure, high temperature), this catalyst makes possible only very low space-time yields. These low space-time yields make economical preparation of aromatic carbonates using such supported catalysts impossible.
Up to now, there has therefore been no supported catalyst available by means of which diaryl carbonates can be prepared economically and efficiently by reaction of an aromatic hydroxy compound with carbon monoxide and oxygen.
It was therefore an object of the invention to find a supported catalyst having high activity and selectivity which allows the economically efficient preparation of diaryl carbonates by reaction of an aromatic hydroxy compound with carbon monoxide and oxygen.